Rubber goods such as tire treads are made from elastomeric compositions that contain one or more reinforcing materials; see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04.
The first material commonly used as a filler was carbon black, which imparts good reinforcing properties and excellent wear resistance to rubber compositions. However, carbon black-containing formulations often suffer from increased rolling resistance which correlates with an increase in hysteresis and heat build-up during operation of the tire, properties which need to be minimized to increase motor vehicle fuel efficiency. Increased hysteresis resulting from the use of carbon black can be somewhat counteracted by reducing the amount and/or increasing the particle size of the carbon black particles, but the risks of deterioration in reinforcing properties and wear resistance limits the extent to which these routes can be pursued.
Over the last several decades, use of amorphous silica and treated variants thereof, both alone and in combination with carbon black, has grown significantly. Use of silica fillers can result in tires with reduced rolling resistance, increased traction on wet surfaces (one of the primary evaluation criteria for tire treads), and other enhanced properties.
In the search for further and/or additional enhancements, alternative or non-conventional fillers have been investigated. Examples include various metal hydroxides and oxides, macroscopic (e.g., 10-5000 μm mean diameter) particles of hard minerals such as CaCO3 and quartz, pumice containing SiO2, micron-scale metal sulfates, as well as clays and complex oxides.
Regardless of the type(s) of reinforcing filler(s) used in a rubber compound, enhancing dispersion of the filler(s) throughout the polymers can improve processability of the compound (rubber composition) and certain physical properties of vulcanizates made therefrom. Efforts in this regard include high temperature mixing in the presence of selectively reactive promoters, surface oxidation of compounding materials, surface grafting, and chemically modifying the polymer(s), often at one or both of its termini. Terminal chemical modification can occur by reaction of a terminally active, i.e., living (i.e., anionically initiated) or pseudo-living, polymer with a functional terminating agent. Terminal modification also can be provided by means of a functional initiator, in isolation or in combination with functional termination. Functional initiators typically are organolithium compounds that additionally include other functionality, typically functionality that includes a nitrogen atom.
Polymers incorporating 3,4-dihydroxyphenylalanine (DOPA) have been synthesized for some time, often for adhesive applications; see, e.g., U.S. Pat. No. 4,908,404. Because these polymers can be costly and difficult to produce, so-called bulk polymers approximating their performance have been pursued. Westwood et al., “Simplified Polymer Mimics of Cross-Linking Adhesive Proteins,” Macromolecules 2007, 40, 3960-64, describe one such process, although the de-protection step employed cannot be used when the polymer contains ethylenic unsaturation. Less restrictive approaches are described in international patent publication nos. WO 2009/086490 and WO 2011/002930, each of which describes the production of functional polymers that exhibit excellent interaction with various types of reinforcing fillers. Enhanced interactivity between polymer chains and particulate fillers typically results in reduced hysteresis as evidenced by reduced tan δ values at elevated temperatures (e.g., 50° to 60° C.).
Polymer chains that include hydroxyl group-containing aryl functionality tend to interact very strongly with other similar chains as well as with hydroxide- and oxide-type particulate fillers, including silica. While the latter can be a desirable trait during use (e.g., after a vulcanizate is prepared from a rubber composition), these interactions can negatively impact processability of the rubber composition itself. More specifically, such rubber compositions can exhibit extremely high compound Mooney viscosity values, a characteristic that increases the amount of energy that must be inputted to ensure proper mixing and processing of the composition.